French Drain Installation

Start your French drain installation by drawing a plan this may help you access the ground layout and make a good sensible decision on the placement of the French drain. (Draw in pencil as the plan may change as you go on).

DIGGING THE TRENCH for your French Drain Installation

Firstly you should be sure that there's nothing under the route you'll be following that could cause problems. Trenches down as little as a single spade's depth find things unexpected and unwanted. Do check the plans and any surveys for gas, electric, telecoms water pipes and other drains. You should really hire a C.A.T. (Cable Avoidance Tool) and instructions, to locate cables and pipes

During the work, keep an eye out for changes in the ground such as tiles that may be protecting something, marker tape, broken drain pipe, disturbed ground or gravel from previous diggings, or even planks that may not be just rubbish.

You will need to work out your preferred fall and depth of your pipe. The fall is the slope of the bottom that allows water to flow downhill the full length of it to the outflow. A suitable fall would be at least 1 or 2 degrees when the pipe gets to the French drain it can sit level. If your land is sloped, just follow that slope. However, if your land is flat, you'll need to ensure that you'll be able to dig deep enough at the outflow point.

The depth will depend on two things - how deep it needs to be for it to be out of the way of your normal cultivation and how deep it needs to be at the bottom of the slope (the point of outflow). Hopefully, it won't get too deep for you to manage otherwise you'll have to call for help.

Your Basic Tool for French Drain Installation

You may need no more than a standard garden spade. In UK these items have a blade with the shaft suitably bent near the blade top to allow easy digging. The handle is a D or T shape.

The blade is more-or-less straight and parallel vertically, slightly curved across (convex at the back and concave at the front), for strength and is usually (280 x 180mm) wide.

Marking Out for French Drain Installation

Mark all the way along the line of your trench about an inch outside of the actual digging line, using pegs and line or spray-on marker paint.


If all or part of your trench is to be in a lawn you'll need to strip the turf from the appropriate area. You will be best marking lines on either side of the route you're taking, leaving about an inch (25mm) each side wider than your spade. Use your garden spade, which is quite satisfactory despite the curve of the blade - just line it up evenly. Otherwise you could obtain a lawn edging tool (edging iron). This is a half-moon shaped blade usually with a turn-over on the top straight edge to accommodate your boot and it has a long straight shaft, with handle. Note that these are rarely robust, their name suggesting their intended use.

They will usually take a moderate amount of wiggling side-to-side (along the blade) for loosening but very little front to back, so get a good one. Use it by lining it up near your marked line, hold it vertically, put your boot on the top of the blade and press down. Withdraw it vertically. Line it up with your marked lines and press down so that it cuts in to near its full depth, cutting each side so that the width of the sod where the turf will come out is a little wider than your spade, say 1/2 inch (10-12mm) each side.

Now cut across, between the lines, at one end of the trench, then once again at about 6 – 8” (150 – 200mm) from the end. Make sure those cuts are deep enough around the whole sod that you're preparing and that they extend a little beyond where you'll be removing the turf, to ensure the earth is cut ready for you to take a horizontal slice, keeping an inch (25mm) or so of earth, from the lawn.

The lawn will quickly recover from those cuts - but not quite so quickly from scuffs and boot damage, so tread lightly while completing the French Drain Installation

The next step assumes you have enough room to work in – treading on your border if necessary. You'll be taking out one small sod at a time, cutting each side and the end for each. You can prepare more than one sod at a time, of course but the slits can be easy to lose in the grass. You'll be working in a forward direction, cutting sods a little wider than your spade, of an easily manageable length along the trench route.

Holding your spade with the blade horizontal and one edge just outside one of the side lines, the other edge inside, push the spade forward an inch (25mm) or so, and then do the same at the line on the other side. Repeat going further in first checking that the spade is horizontal, until you just pass the end cut.

Because you sliced slightly larger than the marked sod and your spade, you can slide the spade underneath it and should be able to lift it out cleanly. Repeat with cuts lengthwise and crosswise as you progress.

Place the sods grass down to store them, either on the lawn or elsewhere, on a tarpaulin if appropriate and well clear of your work area as the French Drain Installation takes a little space. If you lay sod on sod to save space, then lay them grass-to-grass, soil-to-soil for cleaner, easier picking for replacement. The grass under and within the pile will quite quickly go yellow due to lack of light, but will last a week perhaps two, stacked like that, and quickly recover. In hot, dry weather put them in shade and keep them watered.

Now line the French Drain trench with the appropriate lining which is porous a and tough weight it down with some handfuls of gravel. Be sure to leave about a meter over one side of the trench so it can be simply folded over the trench when the gravel is in.

Laying the pipe for a French Drain Installation, start at one end put a heavy weight like a few stones and start to roll it down the trench stopping to put more stones on top of the pipe every 2-3 meters to keep it down be generous with the stones as it will move as you roll. Fill the French Drain trench with the gravel/stones to a about a foot from the surface then fold over the rest of the lining material make sure it is nice and flat. At the end of the job you'll need to replace the earth. Level it approximately, tread it, level it, tread it... until it's a suitable depth below the existing grass then replace the sods in reverse order.

The preferred state of finish is a slight dip rather than a hump (far better would be all dead level, of course, but that's... well, that's just a little unlikely). A hump will not be easy to repair, whereas a dip can be filled in, bit-by-bit, week-by-week, by scattering riddled soil (sieved using a metal garden riddle) and using the back of a rake to move the excess around, filling dips. At first, don't make the scattered soil more than about 1/8" (3mm) deep, on small areas (about the size your sods were). Lightly rub the back of a rake (or something with a straight edge 8 - 12" (200 - 300mm long) along and across the repair - that will even out the soil and expose blades of grass to the light.

The wounds from the French Drain Installation will heal fairly quickly but may take some time to recover to a seamless state. You won't have any patches damaged enough to need reseeding.

Congoratulations you have completed your French Drain Installation.

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